Particularly in motor vehicles, a large number of functions are, even today, initiated or controlled by means of remote controls. Usually, this involves the use of a radio link in license-free frequency bands for the transmission from and to the motor vehicle. For vehicle entry and, by way of example, also for starting the engine, these are what are known as “remote keyless entry” systems (RKE systems for short), as are used for radio central locking, for example. RKE systems have now become the standard solution not only for convenient locking and unlocking of a vehicle but also for further convenience functions. This is done by means of radio control, which is usually integrated in a vehicle key and which, furthermore, is used to activate or deactivate not only the locking and unlocking of the doors and the trunk but also the theft prevention and the engine immobilizer as appropriate. Further functions, such as convenient opening and closing of windows, sun roofs, sliding doors or tailgates may likewise be integrated as well. A further convenience function and safety function is the activation of the headlamp lighting of the vehicle. Additional safety is provided by what is known as an emergency button which is integrated in the key and which, upon being pushed, initiates an audible and visual alarm on the vehicle.
In this case, RKE systems of this kind operate—depending on requirements—with unidirectional or bidirectional communication in the range of worldwide approved ISM frequencies (ISM=Industrial, Scientific, Medical). By way of example, further features are protected data transmission with optionally increased safety as a result of a challenge-response authentication method (bidirectional) and low power consumption. In addition, further applications allow the functions of an RKE system to be personalized for selected people. The range of such RKE systems is usually up to 100 m.
A further radio communication based system is what is known as the PASE system. In this case, PASE stands for PAssive Start and Entry and describes a keyless entry and starting system. In the case of this keyless vehicle entry system, the driver merely needs to carry an identification encoder (ID) and is afforded entry to the vehicle by simply touching the door handle. As soon as the driver is inside the vehicle, the engine can be started by pushing a button. When the driver leaves the vehicle, the PASE system locks the vehicle either automatically or when a button is pushed. The identification pass for the driver replaces conventional mechanical or radio controlled keys and is intended to provide the driver with maximum convenience and the simplest handling. In this case too, there is again the option of personalization for selected people, and multichannel bidirectional data transmission is usually employed which likewise takes place wirelessly and in encrypted form, for example in the range of worldwide approved ISM frequencies.
In addition, systems are also becoming established in the field of motor vehicles today which have further functions such as the transmission of state information. Such systems generally operate over relatively long ranges, usually several 100 m. Examples of these are what are known as telestarting, i.e. starting an engine from relatively great distances, or the remote control of a vehicle heater, an automatic air-conditioning system and so on. Further examples of the use of radio links with relatively long ranges other than those for the described RKE and PASE systems relate to status information for the motor vehicle which can be retrieved from a relatively great distance, such as the current locking state, the current interior temperature and results of technical system checks (engineering checks). It is also desirable for alarm reports to be transmitted over a relatively great distance.
All functions which require wireless data transmission over relatively great distances are also embraced by the generic term “long range applications”. One aim of long range applications is to divide data transmission or communication over distances of approximately 1 km. When (bidirectional) radio links are implemented over such distances, there is, particularly in the USA, the problem that high radio powers (up to 1 watt, 30 dBm) are permissible only for large frequency bandwidths (>500 kHz). When such ranges are implemented in radio locking systems for motor vehicles, however, only relatively small bandwidths (and accordingly low data rates (1 kbit/sec)) are customary, which meets the requirement for high receiver sensitivities. In addition, it is also necessary to implement a high transmission power, however. This is because although small signal bandwidths require low data rates, they result in high bit sensitivities. This is not enough in the link budget, however, since it is additionally also necessary to implement high transmission powers. This is hampered by the regulations in the USA. Spread spectrum technology is the solution in this case. The narrowband (& low-rate) data signal is transmitted in spread (wideband & high-rate) form, the transmission power can be increased (FCC), and in the receiver the relatively low chip sensitivity resulting from the spreading is raised back to the bit sensitivity of a narrowband system by the despreading.